Biomass is quickly becoming an important feedstock for energy generation in power plants. Slagging and fouling are the two main issues with certain biomass fuels due to their composition and nature. From a chemical point of view, most ashes from biomass could be characterized as silicate dominated systems with varying content of basic oxides and a relatively high degree of volatilization of alkali sulfates and chlorides (See D. Boström et al., Conference on Impacts of Fuel Quality on Power Production & Environment, Finland, 2010). Due to the significantly different chemical make-up of biomass fuels (high K2O, high SiO2, and low Al2O3) in comparison to coal, biomass ash typically has a very low melting behavior compared to coal ash (high SiO2, high Al2O3, low K2O) which leads to higher slagging potential. At a high silica and low alumina concentration, it is known in the art that the ash will predominantly form low temperature eutectics where potassium will act as a further fluxing agent (G. Dunnu, J. Maier, G. Scheffknecht, Fuel, 89, 2010, 1534-1540).
The use of additives for the purpose of gaining performance advantages in fuel combustion has been given much attention in the art. Such additives may generally be classified as (1) preflame additives (for improved storage and handling of fuel), (2) combustion additives (for improved combustion efficiency and reduction of pollutants and particulates), and (3) post-flame functioning additives (for particulate collection, fireside deposit control, and cold side corrosion reduction). In the present invention, the emphasis is on class (3) fuel additives, especially those which control the deposits of slag including in commercial-sized furnaces using coal and other solid fuels.
Among known class (3) additives are magnesium-based additives, such as magnesium oxide and metallic magnesium particles, which can be added to slag and slag precursors by spraying or injecting into a combustion chamber. Reduction in deposits in both oil and coal fired boilers has been reported (See U.S. Pat. No. 7,845,292, “Process for Slag and Corrosion Control in Boilers.” Kaolin is another known additive to coal as well as biomass fuels, acting as a slag suppressant. The presence of silica in Kaolin's structure, however, will encourage the formation of potassium silicates which are known to promote slagging (See E. Lindstroem et al., Energy & Fuels 24, 2010, 3456-3461). Gibbsite has also been used as an additive for coal to reduce slagging. Its application is specified for coal rich in calcium and iron, creating calcium-silica-alumina compounds (See Smyrniotis, C. R. et al., “Targeted reagent injection for slag control from combustion of coals high in iron and/or calcium,” PCT Publication Number: WO2010006325).
During combustion in a boiler furnace, reactions involving fuel impurities lead to deposit formation which adheres to boiler surfaces. Such deposits upset the normal operating conditions and produce problems by causing: (a) obstruction to gas flow; (b) interference with heat transfer resulting from the insulative nature of such deposit; (c) damage to water tubes on the ash slopes particularly with bulky deposits; (d) corrosive conditions producing substantial loss of metal tubes. These deposits and associated corrosion force costly nonscheduled outages for cleaning (e.g., deslagging) and/or for replacing the failed tubes.
There thus exists an ongoing industrial need for new and improved class (3) fuel and slag treatment additives to minimize the slagging and deposit producing (i.e., fouling) potentials of biomass and other combustible organic solid fuel combustion products.